An automated imaging system including microscope optics, a camera, a motorized stage, and computer control system can automatically acquire images of cells from a biological specimen deposited on a microscope slide. The system may then process these images to identify cells of interest. These cells may need to be revisited later; for example, to acquire higher-magnification images of such cells or to present certain cells of interest for manual review. This revisiting often occurs on a separate imaging station. For the stage locations corresponding to cells of interest found at the first imaging station, this second imaging station must relocate those exact cells for further review.
If the coordinate systems of the two imaging stations were identical, the second station could simply move its stage to the stage position reported for a cell on the first imaging station, and that cell would be centered in the field of view at the second imaging station. However, due to differences from stage to stage or inaccuracies in how the slide is loaded or secured at a particular imaging station, the same stage coordinates on two different imaging stations often correspond to different regions of the specimen presented in the field of view for the imaging station optics. An error may occur when the slide, upon being placed on a movable stage of an imaging station, is skewed slightly or not banked properly against the slide holder. Further, an error may occur when the nominal zero position of the stage gradually changes from day to day due to inaccuracies in the stage home switches. Small errors of this sort have a dramatic effect because a higher magnification field of view on a second imaging station may be less than a tenth of a millimeter across the specimen. An error of even half that amount is sufficient to displace the cell of interest from the image. Therefore, it is advantageous, for each slide, to measure and correct for any error in slide positioning.
Traditionally, microscope slides used in imaging systems or for manual review have one or more “fiducial marks” printed on them which establish a standard coordinate system for the slide that is independent of the stage. At each of the imaging stations, the imaging system locates these marks on the slide and the differences in their coordinates are used to determine a transformation that can be applied to convert coordinates from the first imaging station into coordinates for the second imaging station. This method is effective, but requires special marks printed on the slides, which potentially increases slide manufacturing costs. In addition, the method is time consuming, because it may be necessary to search several location before the fiducial mark is located in an image. This is especially difficult at high magnification because the field of view is smaller as compared to an imaging station using a lower magnification. There is a need, therefore, for methods and systems that translate stage coordinates for an object of interest or field of view on one imaging station to coordinates for other imaging stations, without the use of pre-printed fiducial marks on microscope slides.